(1) Field of the Invention
The present invention relates to a reference voltage generation circuit composed of MOS transistors.
(2) Description of the Related Art
In recent years, reference voltage generation circuits are used for the purpose of providing stable reference voltage which is not affected by temperature variation and power supply voltage variation. As the reference voltage generation circuit, there are various types of circuits; but a bandgap reference circuit is often used which uses the bandgap voltage of semiconductor material (for example, refer to Japanese Unexamined Patent Application Publication No. 11-45125). The bandgap reference circuit generates stable reference voltage by use of bandgap voltage characteristics of semiconductor material. The bandgap reference circuit will be described below.
The bandgap voltage of semiconductor material is a physical constant at absolute zero temperature; for example, the bandgap voltage of silicon has a value of about 1.24 V. As the temperature of semiconductor material rises from absolute zero, the bandgap energy of semiconductor material decreases and thus a negative temperature coefficient appears. Consequently, the forward bias voltage across PN junction where a P-type semiconductor and a N-type semiconductor are bonded decreases as the temperature of semiconductor material rises, its reduction rate depending on the cross sectional area of the PN junction and the semiconductor material used. As a result, in two PN junctions composed of the same semiconductor material and having a different cross sectional area of PN junction, when the temperatures of the two PN junctions vary, the forward bias voltages across the two PN junctions vary at a different rate. The bandgap reference circuit uses the voltage relationship between these two PN junctions each biased in a forward direction to output reference voltage relatively non-sensitive to temperature.
With reference to FIG. 1, the operation of the bandgap reference circuit will now be described. FIG. 1 is a circuit diagram of a constant voltage circuit using a conventional bandgap reference circuit.
The bandgap reference circuit 100 has, as illustrated in FIG. 1, a current generation circuit 14 and current-voltage conversion circuit 24.
The current generation circuit 14 includes: P-channel MOS transistors MP12 and MP13 constituting a first current mirror circuit; N-channel MOS transistors MN9 and MN10 constituting a second current mirror circuit; diodes D3 and D4; and a resistor 15 of a resistance value R10. Here, current generated by the current generation circuit 14 is calculated. When the Boltzmann constant is k, absolute temperature is T, the elementary charge quantity of electron is q, the junction areas S of the diodes D3 and D4 are S3 and S4, respectively, and the area ratio S4/S3 is N, then drain-source current IP13 of the P-channel MOS transistors MP12 and MP13 is expressed asIP13=(1/R10)·(kT/q)·ln(N)  (1).
The current-voltage conversion circuit 24 includes: a P-channel MOS transistor MP14; a resistor 16 of a resistance value R11; a diode D5; and an operational amplifier 71, and performs a function of converting constant current IP13 supplied from the current generation circuit 14 into voltage.
In the bandgap reference circuit 100 having the above configuration, an output voltage after the current-voltage conversion can be extracted from a node to which the resistor 16 and the drain terminal of the P-channel MOS transistor MP14 are connected. When the voltage of this node is reference voltage (bandgap output voltage) Vref and the forward voltage of the diode D5 is VF, then reference voltage Vref is expressed asVref=(R11/R10)·(kT/q)·ln(N)+VF  (2).
The bandgap reference circuit 100 is characterized by being stable against ambient temperature variation. Thus, the variation of reference voltage Vref with respect to ambient temperature will now be described. The relationship formula between ambient temperature T and the variation of reference voltage Vref is expressed as∂Vref/∂T=R11/R10·(k/q)·ln(N)+∂VF/∂T  (3).In formula (3), when proper values are selected for the resistance of the resistors 15 and 16 and the junction area ratio N between the diodes D3 and D4, there can be obtained reference voltage Vref being output voltage relatively unaffected by temperature. More specifically, when the negative temperature coefficient relating to the PN junction of the diode D5 in the second term in the right-hand side of formula (3) is balanced with the positive temperature coefficient relating to the difference of PN junction in the first term in the right-hand side of formula (3), a reference voltage Vref not affected by temperature can be obtained.
When a circuit composed of transistors and diodes of this type is designed, the characteristics of the transistors and diodes may vary depending on the processes. When the characteristics of the devices vary, the stability of reference voltage may be reduced. Accordingly, when voltage accuracy must be ensured, reference voltage must be calibrated by use of a fuse trimming circuit. Consequently, in the constant voltage circuit of FIG. 1, a fuse trimming circuit 45 is connected to the current-voltage conversion circuit 24. That is, trimming resistors 17 and 18 having resistance values R12 and R13 are arranged as resistors for calibration. When the output voltage of the operational amplifier 71 is Vbgr, voltage Vtrim obtained after fuse trimming is expressed asVtrim={R13/(R12+R13)}·Vbgr  (4).Here, the operational amplifier 71 is an impedance conversion device, and reference voltage Vref and output voltage Vbgr have the same value, exclusive of offset voltage of the operational amplifier 71. As a result, when the resistance values of the resistors 17 and 18 can be varied, calibration can be made for the variation caused by process variations, and voltage equal to or less than reference voltage Vref can be outputted. In this case, the output voltage Vout of the operational amplifier 71 is expressed asVout=Vtrim={R13/(R12+R13)}·{(R11/R10)·(kT/q)·ln(N)+VF}  (5).
Note that, in the constant voltage circuit of FIG. 1, there is arranged an operational amplifier 72 acting as an impedance converter for transmitting output voltage Vtrim to a subsequent stage. However, when the input impedance of the subsequent stage is sufficiently high, the operational amplifier 72 does not need to be arranged.